1. Field of the Invention
The present invention relates generally to radio receivers, and in particular, to a circuit arrangement for amplifying incoming radio frequency (RF) signals through an antenna. More particularly, the present invention relates to an improvement in quality of a reception signal of an AM radio receiver having an automatic channel selecting function.
2. Description of the Background Art
An AM radio receiver is well known which receives an AM (amplitude modulated) broadcasting radio wave (RF signal) transmitted from a broadcast station. One example thereof is described in '88 Sanyo Semiconductor Data Book, "Section: Bipolar Integrated Circuit for Car Audio", P73. The circuit arrangement of such AM radio receiver is shown in FIG. 1.
With reference to FIG. 1, the AM radio receiver includes an RF amplifying circuit 5 for amplifying an incoming RF signal from an antenna 1 and an RF tuning circuit 6 for extracting an RF signal having a desired channel frequency from the RF signal amplified by RF amplifying circuit 5.
RF amplifying circuit 5 includes an antenna damping circuit 2 for attenuating an incoming RF signal from the antenna 1, an FET (field effect transistor) 3 for amplifying the attenuated RF signal and an AGC (automatic gain control) transistor 4 for adjusting a gain of the RF signal amplified by FET 3 and transmitting the adjusted signal. AGC transistor 4 comprises an npn bipolar transistor. The amplification gain of which is controlled by a gain controlling signal applied to its base.
RF tuning circuit 6 comprises an LC resonance circuit including a coil, a capacitor and a variable capacitance diode. The resonance frequency of this LC resonance circuit can be changed by varying the capacitance of the variable capacitance diode in response to a tuning frequency controlling signal voltage VT. RF tuning circuit 6 outputs an RF signal having a narrow band tuned to a desired channel frequency.
The radio receiver shown in FIG. 1 further includes a mixing circuit 8 for mixing the tuned (narrow band) RF signal from RF tuning circuit 6 with a local oscillating frequency signal from an local oscillating circuit 7 to generate an intermediate frequency (IF) signal, an IF amplifying circuit 9 for amplifying the IF signal from mixing circuit 8, and a detecting circuit 10 for AM detecting the amplified IF signal from IF amplifying circuit 9 to output a low frequency signal.
A superheterodyne system including such a frequency conversion allows an amplification at two different frequencies (RF amplification and IF amplification), leading to a stable amplification and high sensitivity. The frequency of the IF signal is fixed (for example 450 Khz) to be lower than that of the reception (desired) RF signal, improving a selectivity.
The difference between the frequency of the oscillating signal and the tuned frequency of RF tuning circuit 6 should be fixed. RF tuning circuit 6 therefore receives the same (or corresponding) control signal VT as the control signal VT applied to oscillating circuit 6, and the tuned frequency varies in response to the oscillating frequency.
A tracking adjustment is carried out for fixing the difference between the tuned frequency F1 of the RF tuning circuit 6 and the oscillating signal frequency F2 of the local oscillating circuit 7. It is difficult to hold a difference constant between the tuned frequency f1 and the local oscillating frequency f2 in all the reception frequency bands.
Hence, in an ordinary AM radio receiver, adjustment is made such that a tracking error becomes zero at specific tracking points (for example at 600 KHz and 1400 KHz) in the reception frequency band (for example from 522 KHz to 1629 KHz).
As a result, a conventional AM radio receiver allows the gain of RF amplifying circuit 5 to decrease due to inaccurate AGC control (the signal VAGC varies with an IF signal level) and selectivity to be deteriorated due to a variation of the IF signal frequency.
An AM stereo receiver further allows a separation to be deteriorated. In an automatic channel selection, a channel selection is carried out based on an IF signal level, whereby the sensitivity for channel selection stopping varies.
Tracking error is minimized in a reception frequency range but it is large of frequencies a band in outside of the tracking points, that is, in 522-620 KHz and 1400-1629 KHz.
In order to eliminate such deviation of the tuning frequency of RF tuning circuit 6, proposed is a non-tuning system AM radio shown in FIG. 2.
In FIG. 2, the AM radio receiver includes an RL parallel circuit 60 which generates a non-tuning RF signal (referred to as a wide band RF signal) from the RF signal amplified by RF amplifying circuit 5, and which parallel circuit constitutes an output load of AGC transistor 4.
The resistor of RL parallel circuit 60 is set to have a relatively large value allowing a sufficiently large signal change to be transmitted to subsequent stage. The radio receiver further includes a first mixing circuit 8 for mixing the wide band RF signal with a first local oscillating signal from a local oscillating circuit 7 to generate a first IF signal of 10.7 MHz, and a second mixing circuit 11 for mixing the first IF signal with a second local oscillating signal from a second local oscillating circuit 12 to generate a second IF signal of 450 KHz.
First local oscillating circuit 7 generates the first local oscillating signal in response to an oscillating signal from an oscillating circuit 70. Oscillating circuit 70 changes an oscillating frequency in response to a control signal VT.
Second local oscillating circuit 12 generates the second local oscillating signal in response to a fixed oscillating signal from a crystal oscillator 12'.
The second IF signal from second mixing circuit 12 is applied to an IF amplifying circuit 9.
The radio receiver of FIG. 2 employs a double conversion system generating first and second IF signals to enhance its selectivity, and no RF tuning circuit is required. Therefore, no tracking error occurs.
In addition, the first IF signal frequency is set as high as 10.7 MHz to significantly reduce image interference.
However, in the radio receiver of FIG. 2, with no RF tuning circuit provided, all the signals including an interference signal within the reception wave band are applied to first mixing circuit 8, thereby significantly deteriorating an interference signal characteristic, in particular, a cross modulation interference characteristic.
FIG. 3 shows a possible arrangement allowing an elimination of tracking error and an improvement in a cross modulation interference characteristic.
In FIG. 3, an RF tuning circuit 6 is provided as a collector output load of an AGC transistor 4 and a resistor 13 is provided between RF tuning circuit 6 and a power supply +Vcc.
Tuning circuit 6 outputs a narrow band RF signal tuned with a desired frequency. Resistor 13 produces a wide band RF signal.
In an automatic channel selection, a channel selection is carried out by using a wide band RF signal. After the channel selection, a reception is made by using a narrow band RF signal.
With this arrangement, the channel selection is carried out by a wide band RF signal to cause no tracking error. A narrow band RF signal is employed in a tuned reception state other than an automatic channel selection to improve a cross modulation interference characteristic.
The arrangement shown in FIG. 3, however, has the following problems to be solved.
In order to extract a wide band RF signal and transmit the extracted wide band RF signal to downstream circuitry at a signal level at which the downstream circuitry does not malfunction, resistor 13 should have a relatively large resistance value, for example, 2k.OMEGA..
When a strong field interference signal, particularly a signal of a shortwave band (2 MHz-30 MHz) outside the band of RF tuning circuit 6 is received, however, high resistance 13 detects the reception of the interference signal to cause a large signal change at the primary coil of RF tuning circuit 6, which change is transmitted to the secondary coil of RF tuning circuit 6. In other words, the isolation of RF tuning circuit 6 is deteriorated to leak the signal from high resistance 13 to RF tuning circuit 6. The output of RF tuning circuit 6 is applied to the first mixing circuit generating a first IF signal of 10.7 MHz. Consequently, the signal leaked from resistor 13 is superimposed on the first IF signal to cause beat frequency interference.